Italki notifies you when the lesson is over. You must click in the button "completed lesson", and after that the site asks you to leave from 1 to 5 stars and write a review. Hope that helps you.

I would assume you either get an email reminder or click on the session once it has ended. There, instead of saying how long there is to go until the lesson starts, it should say leave a review. That's if it's the same as it is for the teacher.